Conversion of mat jack-up drill platforms to floating drill platforms



June 2, 1970 Filed Jan. 10, 1969 J. F. ,HOLMES CONVERSION OF MAT JACK-UP DRILL PLATFORMS TO FLOATING DRILL PLATFORMS 3 Sheets-Sheet 1 "PRIOR ART INVENTOR. JOHN F. HOLMES June 2, 1970 J. F. HOLMES 3,515,084

CONVERSION OF MAT JACK-UP DRILL PLATFORMS T0 FLOATING DRILL PLATFORMS Filed Jan. 10, 1969 3 Sheets-Sheet 2 INVENTOR. JOHN F. HOLMES AGENT June 2, 1970 J. F. HOLMES CONVERSION OF MAT JACK-UP DRILL PLATFORMS TO FLOATING DRILL' PLATFORMS 3 Sheets-Sheet 3 Filed Jan. 10, 1969 T m 9 r v H m 3 m a L A 0 df Cu H F 0 w Z J m United States Patent O 3,515,084 CONVERSION OF MAT JACK-UP DRILL PLAT- FORMS TO FLOATING DRILL PLATFORMS John F. Holmes, Andover, Mass., assiguor to Sanders Associates, 5216., Nashua, N.H., a corporation of Delaware Filed Jan. 10, 1969, Ser. No. 790,274 Int. Cl. B63b 43/04; E02b 17/00 US. Cl. 114-.5 8 Claims ABSTRACT OF THE DISCLOSURE There is herein described a method and apparatus whereby offshore oil drilling apparatus may be converted from shallow to dee water drilling operations. A floatation unit is added to the conventional mat jack-up type drill platform between the working platform and the base mat. The entire floating structure is tuned to a natural period of oscillation about its center of rotation which is longer than the period of the waves which tend to excite the structure.

BACKGROUND OF THE INVENTION Field of the invention 7 The present invention relates to well drilling apparatus and more particularly to offshore oil drilling platforms of the floating type.

Description of the prior art IPrior to the present invention offshore oil drilling has been performed from the so called mat jack-up drill platform. This apparatus comprises basically a base mat to which a plurality of legs are attached and a working platform which may be moved up and down on the legs with respect to the base mat. Once moved into position the base mat is lowered to the ocean floor and the working platform is then jacked-up, out of the water. The greatest disadvantage of this type of drill platform is the limited Water depth in which drilling operations may be conducted. Since the base mat must rest on the ocean floor and the complete working platform must be supported on the legs, water depths greater than about four to five hundred feet generally preclude drilling with the mat jackup platform.

Water depths greater than four hundred feet generally result in a requirement to conduct drilling operations from a floating drill platform and there are two fundamentally different approaches which have been taken to meet this requirement. First is a ship or barge which is modified to permit drilling operations. The modifications which are necessary to convert a conventonal ship hull, barge or catamaran to a fioatng drill platform are typically quite expensive and do not include any provisions to effectively decouple the platform from the motion of ocean waves. Thus the platforms tend to pitch and roll with the waves which results either in breakage and loss of drilling equipment or in a severe limitationupon the maximum sea conditions under which drillng may be conducted. The second approach involves the use of the tuned sphere principles described in co-pending application Ser. No. 665,- 464, entitled Tuned Floating Bodies, filed Sept. 5, 1967 by the same inventor and assigned to the same assignee as the present application. Although the tuned sphere approach is preferred even to the present invention, it is a unique platform which is a complete departure from prior art'floating type drill platforms. Thus the present mat jack-up drill platforms could not be converted to tuned spherical platforms.

OBJECTS AND SUMMARY OF THE INVENTION Thus it is a primary object of the present invention to provide a new and novel method and apparatus for the Patented June 2, 1970 conversion of mat jack-up drill platforms to floating drill platforms.

It is another object of the present invention to provide a new and novel floating mat jack-up drill platform.

It is a further object of the present invention to provide an improved floating mat jack-up drill platform which is decoupled from the motion of the waves.

It is an additional object of the present invention to provide a floating mat jack-up drill platform which permits drilling operations at increased sea states.

It is also an object of the present invention to provide an offshore drilling platform which may be operated in either floating or bottom supported conditions.

The foregoing objects are accomplished through the practice of the present invention wherein a floatation element is added to the existing mat jack-up drill platform structure at a location between the base mat and working platform such that the center of gravity of the entire structure is below the metacenter by a distance corresponding to a natuarl period of oscillation of the structure which is greater than the period of the Waves tending to excite the structure in rotation. The floatation element may be of a lano-section, outrigger pontoon or spherical configuration.

The foregoing as well as other objects, features and advantages of the present inventoin will become more apparent from the detail discussion considered in conjunction with the accompanying drawings. The scope of the invention will be indicated in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of the prior art mat jack-up drill platform to be converted by the present invention;

FIG. 2 is a perspective view of a tuned floating mat jack-up drill platform constructed in accordance with the principles of the present invention;

FIG. 2A is a schematic cross-sectional view of the apparatus of FIG. 2;

FIG. 3 is a perspective view of an alternative embodiment of the apparatus of FIG. 2; and

FIG. 4 is a perspective view of a tuned floating mat jack-up drill platform having a spherical floatation element.

DESCRIPTION OF THE PREFERRED EMBODIMENT Turning now to FIG. 1, there is illustrated in perspective a conventional mat jack-up drill platform which is well known in the prior art. Once this apparatus is moved into position at the drill site, the base mat 10 which may be of a wide variety of configurations is lowered on the legs 12 to the ocean floor 14. The working platform 16 is then jacked-up out of the water supported by the legs 12. The drill derrick 18 is moved into position over the openings 20 and '22 in the working platform 16 and base mat 10 respectively and the drill 24 is advanced into the ocean floor 14. It will be obvious that the stability of this structure is proportional to the length of the legs 12 and the maximum water depth in which drilling operations are possible is thus severely limited.

FIG. 2 shows a perspective view of the apparatus of FIG. 1 after modification in accordance with the principles of the present invention wherein like elements are identified by like numbers. A floatation element 26 is disposed in a preselected plane between the base rnat 10 and working platform 16. The element 26 is of suflicient displacement to float the entire structure. The plane in which the floatation element 26 is disposed is selected such that the center of gravity of the complete structure is at the water-line 28. The mechanics of the apparatus of FIG. 2 are illustrated in the schematic side view shown in FIG. 2A. The center of gravity 30 of the structure is shown in the plane of the floatation element 26 at the water-line 28. The center of rotation of this structure is disposed at a point 32 which is above the center of gravity 30, by a distance which is selected to maintain a natural period of oscillation in rotation which is greater than the period of the waves tending to excite the structure in rotation. A tuned period of approximately thirty seconds is generally suflicient to decouple the structure from the ocean waves. Thus the floating drill platform is mechanically tuned in a manner which is similar to that described in the above-referenced co-pending application without requiring the construction of a completely new spherical drill platform. The natural period of oscillation of the complete structure is related tothe physical structure of the platform by the relationship.

T=21r Wl where T is the natural period of oscillation of the platform; I is the moment of inertia of the platform about the roll axis; W is the displacement of the platform; and l is the distance from the center of gravity to the center of rotation.

It will be apparent that the natural period of oscillation of a drill platform constructed in accordance with the principles of the present invention may be adjusted without disruption of the drilling activities. The floatation unit 26 as stated hereinabove may be moved up and down on the legs 12. This adjustment operates to change the distance between the center of gravity and the center of rotation to a greater or lesser value and thereby increase or decrease the natural period as required to maintain this characteristic of the platform out of the wave spectrum.

In order to minimize the effects of waves laterally incident on the edges of the floatation element 26 it is preferred that the edges thereof be sloped inwardly to thereby cause a substantial portion of the lateral displacement forces to act through the center of rotation 32 of the structure. In this manner the tendency of the waves to excite the structure in rotation is even further minimized.

It will further be obvious that means for mooring any floating drill platform will be required. Since the structure of the invention is mechanically tuned the forces exerted on the structure by mooring lines 31 should also act through the center of rotation 32. Although no practical problems of major significance are encountered in this respect, the angle made by the mooring lines with the vertical should reflect this condition.

A variety of configurations which the floatation element may assume will be apparent. It is within the contemplation of the present invention to provide generally plano floatation elements of any desired transverse crosssectional configuration as well as outrigger pontoon configurations such as shown in FIG. 3 wherein like elements are identified by like reference numerals. It is only required that the physical relationship between the center of gravity and center of rotation be as described with reference to FIG. 2A; i.e. the structure is tuned to a natural period of oscillation which is substantially greater than that of the ocean waves.

A further embodiment of the present invention is illus trated in perspective in FIG. 4. Like elements again are identified with like reference numerals. In this embodiment the conventional mat jack-up drill platform comprising the base mat 10, legs 12 and working platform 16 is modified by the addition of a spherical floatation element 26. The sphere is particularly amenable for use in the practice of the present invention in that, if and when waves wash over the top of the floatation element 26, water run-off is significantly faster with the sphere than with plano surfaces. This feature minimizes any detuning effects which could occur with a plano floatation element during periods of relatively high sea state. In this embodiment of the invention a vertical channel 40 is provided through the floatation sphere 26 to allow passage of the drill 24. The sperical configuration further is ideal from the standpoint of allowing for lateral forces which may be exerted by the ocean waves on the floatation element 26 in that the transverse cross section of the sphere is the same from any direction. Mooring lines again may be coupled to the structure such that the forces exerted thereby act through the center of rotation 32.

It will be understood that the apparatus of the present invention may still be used in a bottom supported condition as a conventional mat jack-up platform in shallow water by merely flooding the floatation unit. Thus this capacity is not irrevocably lost through modification of the platform according to the principle of the present invention.

It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efliciently attained and, since certain changes may be made in the above construction without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

Having described what is new and novel and desired to secure by Letters Patent, what is claimed is:

1. An improved offshore drilling platform apparatus of the type having a base mat and a working platform coupled together by at least one leg wherein said improvement comprises a floatation unit of sufiicient displacement to float said platform disposed between relative to said base mat and working platform such that the center of gravity and center of rotation of the combined apparatus are separated by a distance determined by the relationship where I I is the distance between the center of gravity and center of rotation of said apparatus,

I is the moment of inertia of said apparatus about a roll axis,

W is the displacement of said apparatus, and

T is the natural period of oscillation of said apparatus about said roll axis,

said natural period of oscillation being substantially in excess of the period of waves tending to excite the apparatus in roll.

2. Apparatus as recited in claim 1 wherein said floatation unit comprises a plano section of substantially the same cross sectional configuration as said working platform and aifixed to the legs of said platform.

3. Apparatus as recited in claim 2 wherein the sides of said floatation unit are sloped inwardly and downwardly to thereby direct a substantial portion of the lateral energy of said waves through said center of rotation. I

4. Apparatus as recited in claim 1 wherein said floatation unit comprises a peripheral pontoo structure affixed about the legs of said platform.

5. Apparatus as recited in claim 1 wherein said floatation unit comprises a spherical float affixed t the legs of said platform.

6. Apparatus as recited in claim 1 wherein said floatation unit is movably coupled between said base mat and said working platform such that the distance between said center of gravity and center of rotation may be adjusted to any one of a plurality References Cited of values.

7. Apparatus as recited in claim 1 further including UNITED STATES PATENTS means for mooring said apparatus such that forces 1908,141 10/1959 Marsh X exerted thereby are directed through the center of 9 3 5/1960 Knapp rotation of said appaartus.

8. Apparatus as recited in claim 1 further including JACOB SHAPIRO Prmary Exammer means for flooding said floatation unit to thereby per- U S Cl X R mit operation of said platform in a conventional bottom supported condition. 10 6146.5 

